Amorphous alloys have non-crystalline (amorphous) atomic structures generally formed by fast cooling the alloy from the molten liquid state to a solid state without the nucleation and growth of crystalline phases. As a result of the unique atomic structure produced during this process, amorphous alloys have high mechanical strength and good elasticity, while also exhibiting good corrosion resistance. Therefore, there is strong motivation in the materials field to find new applications for these materials in a variety of industries. However, because amorphous alloys require rapid cooling rates as they are solidified from temperatures above the melting state, it typically has only been possible to produce very thin ribbons or sheets of the alloys on a commercial scale, usually by a melt spin process wherein a stream of molten metal is rapidly quenched.
FIGS. 1a and 1b show partial cross sectional schematic side views of a conventional continuous sheet casting apparatus. In a conventional continuous sheet casting process and apparatus 1, as shown in FIG. 1a, there is an orifice 3 through which molten alloy from a reservoir 5 is injected onto a chilled rotating wheel 7 to form a solidified sheet 9. To provide a steady state flow of melt through the orifice, there are some complex relations that need to be satisfied between the applied pressure (or gravitational pull-down), the orifice slit size, the surface tension of the melt, the viscosity of the melt, and the pull-out speed of the solidification front. In the apparatus shown in FIG. 1a, the pull-out speed of the solidification front is primarily determined by the speed 11 of rotating wheel 7.
As shown, in the detailed view in FIG. 1b, the chill body wheel 7 travels in a clockwise direction in close proximity to a slotted nozzle 3 defined by a left side lip 13 and a right side lip 15. As the metal flows onto the chill body 7 it solidifies forming a solidification front 17. Above the solidification front 17 a body of molten metal 19 is maintained. The left side lip 13 supports the molten metal essentially by a pumping action which results from the constant removal of the solidified sheet 9. The rate of flow of the molten metal is primarily controlled by the viscous flow between the right side lip 15 and solidified sheet 9. In order to obtain a sufficiently high quench-rate to ensure that the formed sheet is amorphous, the surface of the chill body 7 must move at a velocity of at least about 200 meters per minute. This speed of rotation in turn limits the thickness of the sheets formed by the conventional process to less than about 0.02 millimeter.
Although it is possible to obtain quench rates at lower velocities, there are many difficulties that are encountered. For example, at typical melt viscosities and low wheel rotational speeds it is not possible to reliably sustain a continuous process. As a result, the melt may flow too fast through the orifice slit and spill over the wheel, precluding a stable melt puddle and a steady state moving solidification front. Although, some remedies can be implemented, such as reducing the orifice slit size, generally this is not a practical solution because the molten metal would erode the opening of such a small orifice very quickly. Despite these problems, an amorphous metal sheet having a sheet thickness ranging from 50 to 75 μm, and also retaining the mechanical properties of the amorphous alloys is disclosed in U.S. Pat. No. 6,103,396; however, the thickness range available for the disclosed process still leads to limitations in the types of applications in which such materials may be used.
Accordingly a need exists for a continuous process to cast thick sheets of bulk solidifying amorphous alloys.